Major Depressive Disorder is one of the most common psychiatric disorders, affecting millions of people worldwide. Antidepressant drugs are widely used, but 50% of depressed patients receiving these medications will relapse, and 30% are drug resistant. Thus, new targets for antidepressants are needed. Converging evidence indicates an important role for depression-associated changes in hippocampal circuitry. For example, depressed humans and animal models of depression have smaller hippocampi, and decreased activity-dependent genes and processes, like hippocampal dentate gyrus (DG) neurogenesis. Notably, antidepressant drugs and non-pharmacological treatments like electroconvulsive treatment ameliorate these changes. From such work, a new framework for discovery of novel antidepressants has emerged: find treatments that recalibrate depression-linked dysfunctional neural circuits and behavior. Indeed, other approaches to stimulate neural circuits - such as deep brain stimulation (DBS) - can reverse depression-related symptoms and neuropathology, particularly in the hippocampus. Strikingly, in the context of depression, DBS has only been targeted to non-hippocampal brain regions. DBS of the main hippocampal input - the entorhinal cortex (Ent) via the perforant path (PP) - has proven physiologically beneficial in other contexts: it enhances memory in humans and memory and DG neurogenesis in laboratory animals. However, it remains unknown if stimulation of the PP will produce antidepressant-like behaviors. For this exploratory and developmental R21 project, we will test the hypothesis that controlled activation of PP to DG is antidepressive. This hypothesis emerges from our pilot data showing: a) stress and depression are linked to greater PP levels of TRIP8b, a brain-specific auxiliary subunit of HCN channels; b) germline knockout of TRIP8b - which is antidepressive - increases DG neurogenesis; and c) PP disinhibition (de-repression of Ent neuronal excitability via knockdown of TRIP8b) increases DG neurogenesis and is antidepressive. We also provide d) feasibility data that we can chemogenetically-stimulate DG via PP activation. Based on these data, we will: Aim 1: Determine whether molecular-based PP disinhibition promotes antidepressive behavior using viral-mediated KD of TRIP8b in the PP and a battery of depression-linked and cognitive behaviors; and Aim 2: Determine whether chemogenetic stimulation of PP activity drives antidepressive behavior using DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) to mediate neuronal activity and behavior. While the aims proposed here involve considerable risk, they are logical and highly feasible given the large amount of pilot data and the published ability of PP stimulation to improve hippocampal cognitive function. This project is in an early and conceptual stage, and has focused, behavioral outcomes, making it ideal for the R21 mechanism. Even if the hypothesis is incorrect, the experimental design ensures that the collected data will advance our knowledge of brain circuits contributing to affect and cognition.